This invention relates to a process for recovering usable gas from refuse or garbage. In particular, it relates to an improved process for recovering usable fuel gas from refuse or garbage by pyrolysis.
In methods of processing refuse or garbage, the garbage is pressed into pellets or granules that are dehydrated and placed in a heated reactor in which gas is evolved by pyrolysis. Some of the gas is burned to maintain heat in the reactor, and the rest is separated from residual substances such as ash and other noncombustibles. The unburned gas is then piped to a gas converter where it is mixed with air over a glowing coal bed and converted to water gas. A method of and means for performing such a process is described in German Patent Specification DE-OS 33 47 554, which is U.S. Pat. No. 4,557,204. Both of these patents are assigned to the assignee of the present invention.
The process of the subject invention produces usable gas from garbage without generating noxious byproducts except for a relatively high percentage of ammonia in the waste water. The process is also self-sustaining in that it operates on less than half of the available energy of the constituents of the garbage.
The gas thus produced can be used to drive gas turbines and gas engines. Pyrolysis is performed in a deficiency of oxygen in a temperature range under 600.degree. C. to prevent dispersion and subsequent oxidation of the heavy metals present in the garbage. This minimizes the probability of release of heavy metals to the environment, which may be hazardous to people. However, pyrolysis in this temperature range leaves a residue with a relatively high carbon component, which is not available to produce energy in the pyrolysis apparatus. Depending upon the amount of vegetable matter in the refuse and the preseparation of inert materials, the amount of carbon in the pyrolysis residue can be more than 40%.
Since about 5% by weight of household garbage is soluble in water and, therefore, can be treated by microbes to change the carbon-amount into gaseous methane, it has been proposed in U.S. Pat. No. 4,289,625 to treat organic refuse first in a biogas generation zone, and then to thermally treat the residue for additional gas generation. This method has been proposed due to the fact that biogas, which has been recovered by the anaerobic treating of the water-soluble organic dry-substance of the garbage, has more heating energy than pyrolysis gas, i.e. about 23 MJ/Nm.sup.3 compared to about 6 MJ/Nm.sup.3. (The term "Nm.sup.3 " means cubic meter normal: 1013 millibar, 0.degree. Celsius.)
However, such a method does not allow optimal specific treating of the different components and materials from the garbage.
Since household garbage has about 30% by weight organic dry-material, of which only about 17% by weight can be treated anaerobically and in addition contains about 33% by weight inert dry material, such a method also has the disadvantage that a lot of solid residue must be transported through the whole apparatus for a very long time. This means that all parts of the apparatus, including all energy suppliers (heating apparatus, for example, for heating from 33-37.degree. C. up to pyrolysis temperature), have to be dimensioned very large, which makes the whole method and apparatus uneconomical.
A further disadvantage is that in the acid phases of the anaerobic treating, components and materials containing heavy-weight metals (for example, of the group of cellulosic-metal, hemicellulosic-metal and lignin-metal treating products) will be brought into solution and give problems with pollution. Disadvantageous is also the fact that the high amount of non-biodegradable organic materials reduces the speed of reproduction of the micro-organisms and, therefore, the treating process requires a longer time and, therefore, it is further necessary to provide larger dimensions for all apparatus.